PHOENIX – Just over six months after having back surgery, Steve Trachsel will take the mound tonight for his season debut.

Trachsel, who’s been out all year after having surgery on March 19 for a herniated disc, will make his first start of the year as the Mets open a three-game set against the Giants. The 34-year-old right-hander, who was not available yesterday because he left early for San Francisco, made six rehab starts in the minors.

Manager Willie Randolph said that Trachsel should be able to provide normal length tonight.

“He’s been stretched out pretty good,” Randolph said. “He’s been on a couple rehab assignments and he feels strong and his back feels good. We’ll watch his count, but it won’t be like he’ll be on a 60-pitch count or anything like that.

How comfortable Trachsel appears on the hill could go a long way in determining how long he stays in the game.

As Randolph noted, “a lot depends on how he feels.”

Currently the Mets are operating with a six-man rotation, but Randolph said yesterday that unless somebody gets hurt, the plan is to cut it to five. He’s not really in favor of a six-starter plan.

“No, because it throws too many things out of whack,” Randolph said. “And if you do that, it’s going to shorten Pedro [Martinez] and maybe [Tom] Glavine’s turns and I don’t want to do that. You want your best pitchers to pitch in a pennant race. You don’t want to lengthen their time away from taking the ball. You want to give them the ball more actually.”

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Doug Mientkiewicz’s third rehab game for St. Lucie was rained out yesterday. … Contrary to an internet report, Kris Benson – who was pushed back two days in the rotation to accommodate Trachsel – said yesterday that his shoulder is fine. … Mike Piazza (broken bone in left hand) was able to throw on Wednesday and again yesterday and might attempt swings this weekend.

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Kazuo Matsui, who was hit by a pitch on the right upper leg on Wednesday, said yesterday he was still hurting but able to play. Asked after Wednesday’s game if he thought the hit-by-pitch was intentional, he replied, “Obviously you’re going to have to ask them if it was intentional, but it was right on the money. It was two outs. I think that played in the situation.”